The present invention relates to the forming of cylindrical metal tubular articles from a welded pipe blank. The present invention provides a unique process for forming an elongated, relatively thin-walled cylindrical tubular article, with enhanced metallurgical characteristics, from a welded pipe blank of considerably greater thickness. The present invention operates on the pipe blank by room temperature roll extrusion to provide a simultaneous reduction of wall thickness and elongation of the tubular wall structure.
It is well known in the art to produce thin walled metallic pipe of a desired length by roll extruding a relatively thick pipe blank into a thin walled pipe of increased length. Such pipes have been formed from wrought or cast seamless pipe blanks. When wrought or cast pipe blanks are utilized,they are formed by various room or elevated temperature processes including roll extrusion into a final seamless thin walled pipe.
On the other hand, when welded pipes fabricated from thick or thin plate stock are utilized as starting blanks, the final result is a thick or thin walled pipe with the seam being defined by the weld; however in the processing of such welded pipe, roll extrusion is not utilized.
Thin walled seamless pipe has superior characteristics, such as uniform strength, corrosion resistance and the like when compared to the thin walled welded pipe. However, there is a significant cost differential between the two, with the seamless pipe being substantially more costly to fabricate than the welded seam pipe. The extra cost is attributable to the higher cost for wrought or cast seamless blanks relative to welded blanks. The present invention is directed to a unique process for forming an essentially seamless pipe from a welded seam pipe blank.
Thus in the present invention the welded pipe blank is processed such that the microstructure of the weld in the pipe becomes substantially obliterated by complete metallurgical recrystallization and chemical homogenization such that it is substantially not distinguishable from the parent metal of the original plate stock. This is essentially seamless or seam free pipe with a wrought equiaxed grain structure. This is accomplished through the application of thermal processing and of controlled mechanical deformation at room temperature of the original welded pipe blank.
The roll, extrusion step where noted above can be performed by apparatus and techniques well known in the tube or pipe forming art. For example the roll extrusion step can be performed by the method and apparatus shown and described in U.S. Pat. No. 3,222,905 issued Dec. 14, 1965 to A. W. Ernestus for "Method Of Forming Tubular Metal Products By Extrusive Rolling". The disclosure of that patent is incorporated herein by reference.
Thus, the present invention provides an improved method of fabricating substantially seam free metal pipes from welded pipe blanks which in comparison to metal pipes as presently made from welded pipe blanks have greatly improved mechanical strength and toughness, and increased corrosion resistance. This is due to the enhanced microstructural uniformity and the elimination of the mechanical and metallurgical notch concentration effects of the weldment.
Thus, it is an object of the present invention to provide a unique process for making a welded metal pipe which is essentially seamless or seam free.
It is another object of the present invention to make such a seamless or seam free pipe from welded pipe stock with the process substantially reforming the weldment seam to have a microstructure substantially the same as the parent material.
Other objects, features, and advantages of the present invention will become apparent from the subsequent description and the appended claims, taken in conjunction with the accompanying drawings, in which: